Albert Barnes Commentary Matthew 24:14

"And this gospel of the kingdom shall be preached in the whole world for a testimony unto all the nations; and then shall the end come." — Matthew 24:14 (ASV)

And this gospel—shall be preached in all the world. The evidence that this was done is to be derived chiefly from the New Testament, and there it is clear. Thus Paul declares that it was preached to every creature under heaven (Colossians 1:6, 23); that the faith of the Romans was spoken of throughout the whole world (Romans 1:8); and that he preached in Arabia (Galatians 1:17), and at Jerusalem, and round about to Illyricum (Romans 15:19).

We also know that he traveled through Asia Minor, Greece, and Crete; that he was in Italy, and probably in Spain and Gaul (Romans 15:24–28). At the same time, the other apostles were not idle, and there is full proof that within thirty years after this prophecy was spoken, churches were established in all these regions.

For a witness unto all nations. This preaching the gospel indiscriminately to all the Gentiles will be a proof to them, or a witness, that the division between the Jews and Gentiles was about to be broken down. Until now, the blessings of revelation had been confined to the Jews. They were the peculiar people of God. His messages had been sent to them only. When, therefore, God sent the gospel to all other people, it was proof, or a witness to them, that the peculiar Jewish economy was at an end.

Then shall the end come. The end of the Jewish economy. The destruction of the temple and city.